The present invention relates to the field of transmission mechanisms for use in automatic gearboxes, and more particularly relates to such a transmission mechanism which provides various speed stages by use of several selectively engagable power transmission systems each of which is equipped with a clutching mechanism.
There is a known type of transmission mechanism for providing various speed stages between a first rotating member and a second rotating member, such as the crankshaft of an internal combustion engine and a power input member of a differential device of an automotive vehicle, which comprises several power transmission systems, each of which can selectively be engaged to provide transmission of rotational power at its own particular speed ratio (or typically any selectable one of a plurality of speed ratios) between the first and second members, and each of which has a clutching mechanism which either can be connected so as to thus transmit rotational power or can be disconnected so as not thus to transmit rotational power. These power transmission systems may each include a per se well known type of synchromesh device. By appropriately controlling the engagement and disengagement of the various clutching mechanisms, and concurrently controlling the selection of the various speed ratios of the synchromesh devices of the various power transmission systems, which may be done automatically by a transmission control system of a per se well known type so as to provide entirely automatic operation, it is therefore possible to automatically shift between a plurality of speed stages and to provide both powered and engine braking operation of the transmission as a whole.
A particular advantage of this form of transmission system is that it has in principle allowed the vehicle designed to dispense with the use of a fluid torque converter. This has eliminated the relatively great frictional loss inherent in the use of such a fluid torque converter. Further, since a synchromesh type transmission unit which can provide two different speed stages is much lighter and more compact than a planetary gear mechanism which also can provide two different speed stages, such a transmission system as outlined above is much more compact than, and is lighter than, a conventional type of full automatic transmission utilizing a torque converter and planetary gear mechanisms. Therefore during use a vehicle incorporating such a transmission system provides better fuel economy and performance.
However, this prior art type of transmission mechanism has suffered from the disadvantage that such a mode of operation requires very close and accurate control of the timing of the engagement and disengagement of the various clutching mechanisms, especially during starting off of the vehicle from rest. During starting off of the vehicle from rest, if the engagement of the one of the clutching mechanisms relating to the one of the power transmission systems which provides the first speed stage is too swift and positive, then the vehicle will tend to perform so called "jackrabbiting", which involves very jerky starting off from rest, may cause stalling of the internal combustion engine of the vehicle, and may also damage the various components of the transmissionn such as said clutching mechanism, for example. On the other hand, if the engagement of the one of the clutching mechanisms relating to the one of the power transmission systems which provides the first speed stage is too slow and mushy, then the vehicle will tend to start off very slowly and uncertainly, thus providing poor vehicle performance. Such a thusly necessitated very close control of the timing of the engagement and disengagement of the clutching mechanism relating to the one of the power transmission systems which provides the first speed stage has presented a severe disadvantage with respect to the realization of an automatic transmission incorporating such a transmission mechanism, because such close timing control is very difficult to ensure, especially after the transmission has been used for a long period of time and its operational time constants have altered with usage. For example, in the case of a hydraulic type transmission control system, changes in the viscosity of the hydraulic fluid therein and changes in the size of the various orifices of the control system can cause timing difficulties.
Further, difficulties also occur with regard to the accuracy of timing control required for the engagement and disengagement of the various clutching mechanisms, during shifting between the speed stages of the transmission mechanism. If the engagement of one of the clutching mechanisms occurs somewhat too late after the disengagement of the previously engaged transmission mechanism, then for a certain intermediate period the engine associated with the transmission will race, especially during a power on shift as during an acceleration type upshift of the transmission or during a kick down type downshift, and on the other hand if the engagement of one of the clutching mechanisms occurs somewhat too early before the disengagement of the previously engaged transmission mechanism is complete, then for a short time period very large torsional strains are liable to be put on various elements of the transmission, in consequence of such an attempt, effectively, to engage two different speed ratios at one time, and this can damage various elements of the transmission mechanism. Especially, good timing control is important in the case of a power on downshift of the transmission or a so called kick down, when the engine load is high, and snappy and accurate shifting of the speed stages is particularly important and difficult in such a case.
Another factor that is required to be taken account of in the design of a transmission mechanism for an automatic transmission is mountability in a vehicle. The friction engaging elements of a transmission such as the hydraulic clutches thereof are liable to be quite large in diameter, especially as compared with the diameters of the various gear wheels of the transmission, and especially in the case of a front engine rear wheel drive type of vehicle such as transmission mechanism as outlined above has presented the constructional difficulty that the amount of space available for mounting such large diameter clutching mechanisms has not been sufficient, since typically the rear end of an automatic transmission which utilizes such a transmission mechanism has been required to be housed under the floor of the passenger compartment of the vehicle, perhaps in a so called transmission tunnel thereof. On the other hand, in the case of a front engine front wheel drive type of vehicle such a transmission mechanism as outlined above has presented the constructional difficulty that the amount of axial space available for mounting the transmission has been very limited, which limits the axial length of the transmission as a whole.
Finally, it is almost a required characteristic of such a transmission mechanism that it should provide engine braking for the vehicle, at least in some of the speed stages thereof. Smooth starting off from rest of the vehicle, both forwards and backwards, are also very important, and it should not be necessary for unduly delicate control of the clutching mechanisms to be exerted in order to obtain this effect. Power loss in the transmission should be minimized. Further, within these above mentioned constraints, it is a general design objective of such an automatic transmission mechanism to be as light and compact as possible.